1. FIELD OF THE INVENTION
The present invention relates to a head position control system for a disc storage unit in which reference information is read out from a disc-shaped storage medium through a head to detect a position of the head so as to control the position.
2. DESCRIPTION OF THE PRIOR ART
In the disc storage unit of the type described above, the head must be displaced from one track to a specific track and must be properly positioned in relation to this specific track or data. As is well known in the art, in order to shift and properly position the head in relation to the specific track of data at a particular radius on the disc, open and closed loop systems have been used. In case of the open loop system, only a signal representative of a shift distance of the head such as a predetermined number of pulses is applied to a stepping motor and the present position of the head is not detected at all so that the position control in a strict sense is not carried out. The open loop system has been widely used in the case of a disc storage unit in which it is not necessary to precisely position the head in relation to a specific track of data at particular radii of the disc.
A data-packaging density of a disc storage unit, however, has been greatly increased recently so that a pitch between adjacent tracks becomes narrower. As a result, various problems arise when the head is even slightly deviated from its correct position, and thus demand for a clsoed loop control system has increased.
In the case of the closed-loop position control method, the present position of the head must be detected and applied as an actual value to a position control circuit. For this purpose, reference information or servo information to be used to detect the position of the head must be written on the disc. Therefore, the closed loop system is sometimes called a servo system. The servo system includes a servo-surface-servo system and a data-surface servo system.
In the case of the servo-surface-servo system, one surface of a storage disc is exclusively used for the storage of reference information so that information representative of the present position of the head can be read out from the reference information storage surface at any time independently of the operation for writing or reading out data onto or from the other surface of the disc. As a result, in response to the reference information stored on the one surface of the disc, the position of the head can always be corrected so that the head can be properly positioned in relation to a specific track of data. This system, however, has an inherent defect in that the data storage capacity is considerably reduced, since one of the surfaces of the disc must be exclusively used for the storage of reference information as described above.
On the other hand, in case of the data-surface-servo system, reference information is written only into one portion of each track of data in the circumferential direction. Therefore, unlike the servo-surface-servo system, the head position information cannot be read out at any time, but in general it suffices to correct the position of the head in response to the reference signal read out every time when the disc makes one rotation. Therefore, the data-surface-servo system has the distinct advantage that the space for the storage of reference information on the surfaces of the disc can be reduced and accordingly the data storage capacity need not be greatly reduced.
The present invention is directed to the data-surface-servo system of the type described above.
However, the prior art data-surface-servo system has a relatively long seek time required for shifting the head from one track to a specific track and then positioning it properly at a specific track position, so that an access time required for finding a desired track for the writing or reading of data becomes longer, as will be described in detail hereinafter with reference to FIGS. 1 and 2.
FIG. 1B is a developed view showing one track 1a on the disc and reference information is written into a hatched portion thereof. A period of rotation of the disc 1 is T and the direction of the rotation thereof is indicated by P. FIG. 1B corresponds to one of the most typical servo-surface-servo systems or a so-called index burst system in which the reference information 1b is written at one position at the end of the period of the rotation T. In the remaining region 1f corresponding to the remaining time interval of the period of the rotation T left after the reference information stored, there are provided, for instance, 32 data storage sectors defined and spaced apart from each other by a suitable gap in the longitudinal direction of the track 1a. Each sector includes of course a data area into which required data is written or from which required data is read out, and an area for the storage of formatting data, an area for the storage of sync data for a read/write circuit and a margin area.
FIG. 1A shows an index pulse IDX in response to which the reference information 1b is written and the data is written into or read out from the region. 1f. The index pulse IDX is synchronized with the rotation of the disc and is generated during the period of rotation T by a spindle motor 3 shown in FIG. 4 for rotating the disc.
FIG. 2 shows a flow of the steps of the seek operation in the above-described servo-surface-servo system. The operation is started when a disc storage unit such as a fixed disc unit receives a seek command from a computer at step S1. For instance, the seek command is in the form of the number of tracks between a starting track along which the head is positioned and a specific track of data to which the head must be shifted and properly positioned. In response to the seek command, a predetermined number of stepping pulses are applied to the stepping motor for rotating the disc so that the head is shifted at step S2. In the subsequent step S3, suitable means is used to judge whether or not the head is shifted to the specific track of data and the steps 2 and 3 are repeated until the judgement becomes affirmative, i.e., "YES". Even after the heat has been brought to the specific track 1a, a hunting phenomenon occurs for a while before and after the head reaches the specific track, so that next step S4 is provided for settling down the hunting.
At step S5, the reference information 1b is read out after the hunting has been competely settled down and in the succeeding step S6, a deviation of the head from a proper or correct position, i.e., an amount of off-track is detected and then a judgement is made whether the detected deviation or off-track is within a predetermined tolerance band or not. If the deviation or off-track is in excess of a predetermined tolerance band, the position of the stepping motor is corrected at step S7 and thereafter the operation returns to step S5. In general, steps S5, S6 and S7 are repeated a few times and only after the off-track is eliminated will the operation proceed from step S6 to step S8. At step 58, the signal representative of the completion of the seek operation is generated and delivered to the computer. Thus, the seek operation is completed.
In the fixed disc unit, a time required for completing the seek operation is equal to an average access time required for the head to travel 1/3 of one whole track, for instance, about 100 msec. The problem resides in the time required for reading out the reference information 1b at step S5. When the rotational speed of the disc is 3600 RPM, the sampling period for reading out the data once per rotation of the disc is about 17 msec. In addition, as described above, steps S5-S7 are repeated two or three times and accordingly the step 5 alone requires 34-51 msec. In other words, a time equal to about 1/3--1/2 of the seek time is used only in the step for reading out the reference information. This is the main reason why the conventional data-surface-servo system requires a substantially long access time for reading and writing information.
In order to eliminate the defect that the access time is long, the time required for completing step S5 can be reduced to 1/n by writing n pieces of reference information into a plurality of positions, respectively, in the circumferential direction of the track. However, the storage capacity is reduced or sacrificed accordingly. In addition, the reference information must be inserted between the data storage sectors and as a result, the data format of the tracks and a write-read circuit are complicated. In view of the above, the format called soft sector in which all sectors are defined continously as described above is advantageous, so that it is preferable to use the reference information in the above-described system as much as possible.